Update collision pipeline docs to make clear that `CollisionWorld::add(...)` doesn't return a `CollisionObjectHandle`

Hey, thanks a bunch for NCollide. I've used it via other crates for a while and am attempting to use it directly for the first time today.

The collision pipeline docs seem to indicate, both explicitly and implicitly in variable names, that `CollisionWorld::add(...)` returns a handle. I spent a while today wondering why I couldn't stash my handle and refer to it later, use it for updates, etc.

Looks like the return type has changed and the docs/examples in them haven't been updated. I eventually found the `.handle()` method, but not before spending some time wondering if I was missing a conversion somewhere, wondering if I should use something other than a handle, etc.

Thanks.